This invention relates to code division multiple access (xe2x80x9cCDMAxe2x80x9d) mobile communications systems and more particularly to a CDMA mobile communications system with improved channel estimation and pilot symbol transmission.
Various standards for third generation of mobile wireless communications system have emerged recently. These third generation mobile wireless communications systems are designed to provide a wide range of multimedia wireless communications services including full motion video, video conferencing and Internet access. These systems use modulation schemes such as wideband code division multiple access (xe2x80x9cW-CDMAxe2x80x9d) to support data transfer rates up to 2 megabits per second (Mbps).
Code division multiple access (xe2x80x9cCDMAxe2x80x9d) allows simultaneous transmission of information over a common channel by assigning each of the transmitted signals a unique code during transmission. In a wireless environment, a signal transmitted from one point to another arrives via many paths. This is because the transmitted signal bounces off anything, such as the ground, the air, mountains, buildings. etc., it comes into contact with. These multiple signals eventually arrive at some received point where they all add and subtract with each other, resulting, at times, in deep xe2x80x9cfadesxe2x80x9d where there is very little signal strength. In some cases, the reception may be completely disabled. Furthermore, the multipath phenomenon is variable and unpredictable depending upon the locations of the transmitter and receiver. The multipath also changes with time as the environment changes.
However, by spreading the information energy over a wide bandwidth at the transmitter, and by despreading, that is, efficiently collecting the spread information energy at the receiver, signal fading due to multipath may be diminished. In general, the wider the bandwidth, the more resistant the signal is to multipath fading. Thus, while multipath signal fading may cause the narrow band cellular type signal (such as frequency division multiple access (xe2x80x9cFDMAxe2x80x9d) or time division multiple access (xe2x80x9cTDMA)xe2x80x9d) to be diminished considerably, only a very small portion of the W-CDMA energy is lost. Although this unique communication scheme has been known for several decades, the dramatic cost reduction brought about by the advent of integrated digital high speed custom components has recently made commercialization feasible.
A CDMA spread spectrum (xe2x80x9cSSxe2x80x9d)signal is created by modulating the radio frequency (xe2x80x9cRFxe2x80x9d) signal with a spreading sequence (a code consisting of a series of binary pulses) known as a pseudo-noise (xe2x80x9cPNxe2x80x9d) digital signal because they make the signal appear wide band and xe2x80x9cnoise likexe2x80x9d. The PN code runs at a higher rate than the RF signal and determines the actual transmission bandwidth. The resulting signal has a low-power spectral density in any narrow portion of the band. Messages can also be cryptographically encoded to any level of secrecy desired with direct sequencing as the entire transmitted/received message is purely digital.
An SS receiver uses a locally generated replica pseudo noise code and a receiver correlator to separate only the desired coded information from all possible signals. A SS correlator can be thought of as a specially matched filter, i.e., it responds only to signals that are encoded with a PN code that matches its own code. Thus the SS correlator (SS signal demodulator) can be xe2x80x9ctunedxe2x80x9d to different codes simply by changing its local code. The correlator does not respond to man made, natural or artificial noise or interference. It responds only to SS signals with identical matched signal characteristics and encoded with the identical PN code. All CDMA users can thus share the same frequency channel because their conversations are distinguished by a unique digital code. Not surprisingly, this communications method is inherently private, as opposed to the current non-secure cellular radio scheme. Furthermore, the occupied bandwidth (10-20 MHz) and signal handling capability is higher than any other proposed modulation system including the narrow band N-CDMA system standardized (ITU standard IS95) for cellular radio (1.25 MHz) now being deployed primarily for voice services. Thus, the performance of W-CDMA in a wireless environment is far superior to any existing modulation scheme.
For coherent wireless communications, pilot symbol assisted or pilot channel assisted channel estimation schemes may be used. The pilot assisted methods allow a mobile station to acquire the timing of the forward CDMA channel (or a base station to acquire the tuning of the reverse CDMA channel), provides a phase reference for coherent demodulation, and provides a means for signal strength comparisons between base stations for determining when to handoff.
In a pilot symbol assisted method, a known periodic pilot symbol is spread and then inserted into the unknown spread data sequence, as shown in FIG. 1A. A channel estimating technique such as a simple averaging of the pilot symbols may then be used to provide a channel estimate used in maximal ratio combining at a rake receiver, not shown, as illustrated in FIG. 2. In the rake receiver, the signals of several correlation receivers belonging to the strongest multipath components are combined to provide an enhanced, higher quality signal.
In a pilot channel assisted method, the known pilot signal and the unknown data sequence are separately spread using two uncorrelated spreading sequences at the same chip rate. A exemplary frame structure for an exemplary pilot channel assisted estimation scheme is shown in FIG. 1B which includes a separate data channel 12 and pilot channel 15. Each frame 19 of length Tr=10 milli-seconds (ms) of pilot channel 15 information is split into N=16 slots 17, each of length Tslot=0.625 ms. Within each slot, the data channel 12 and pilot channel 15 transmit information in parallel although they may be at different rates, i.e., have different spreading factors.
An exemplary pilot symbol assisted scheme, on the other hand, as in FIG. 1A, proposes to insert four (4) pilot symbols in every time slot for 32, 64, and 128 kilo-symbol per second (KSPS) data channels and for the C4 KSPS downlink control channel and eight (8) pilot symbols in every time slot for data rates of 256, 512, and 1024 KSPS. The overhead associated with transmitting the pilots in each time slot is summarized in Table 1 hereinbelow.
What is needed is an efficient channel estimation scheme which requires a minimal amount of pilot information.
The present invention includes an improved system and method for transmitting and receiving digital information over mobile communication channels. The present invention includes an enhanced channel estimator which iteratively estimates channel amplitude and phase distortion from received pilot and data signal information at various time instants.
An object of the interactive channel estimation scheme of the present invention is to allow for transmission of a minimal amount of pilot information.
Another object of the iterative channel estimation scheme of the present invention is to provide for improved performance while transmitting the same number of pilot symbols.
These and other features of the invention that will be apparent to those skilled in the art from the following detailed description of the invention, taken together with the accompanying drawings.